The purpose of this research study is to study how patients with liver and kidney transplants absorb, and eliminate the drug mycophenolate mofetil (Cellcept). This is a medication used to prevent organ rejection in patients receiving various organ transplants and is a drug the subject will be or have been placed on. This study will not change treatment. Instead, it will collect extra blood and (for some subjects) bile, to study how Cellcept is handled by the body.